Interferometric measurements using phase shifting techniques are presently capable of subnanometer resolution. A number of currently-available products are utilizing this technology to provide fast, non-contact, and highly repeatable profiles of object surfaces and topological features. See, for example, J. F. Biegen et at., "High-Resolution Phase-Measuring Laser Interferometric Microscope For Engineering Surface Metrology", 1 Surface Topography 469 (1988). It is well known, however, that because of phase ambiguities, surface features with relative height displacements or discontinuities that exceed +.lambda./4 between adjacent measurement sites are only determinable to a resolution of modulo .lambda./2, where .lambda. is the average wavelength of the illumination source.
A number of methods have been proposed and implemented to overcome this limitation in the topological profiling of such so-called rough surfaces. Among these are multiwavelength methods such as described by K. Creath, "Step Height Measurement Using Two-Wavelength Phase-Shifting Interferometry", 26 Appl. Opt. 2810 (1987), and by Y. Y. Cheng and J. C. Wyant, "Two-Wavelength Phase-Shifting Interferometry", 24 Appl. Opt. 804 (1985), coherence scanning methods such as described by G. S. Kino and S. S. C. Chim, "Mirau Correlation Microscope", 29 Appl. Opt. 3775 (1990), and by B. S. Lee and T. C. Strand, "Profilometry With A Coherence Scanning Microscope", 29 Appl. Opt. 3784 (1990), and order counting methods such as described by T. C. Strand and Y. Katzir, "Extended Unambiguous Range Interferometry", 26 Appl. Opt. 4274 (1987).
Multiwavelength schemes (see, for example, U.S. Pat. No. 4,832,489 to Wyant et al. and U.S. Pat. No. 5,127,731 to DeGroot) combine the measured phases from several illumination source wavelengths to produce a phase map corresponding to light of an equivalent wavelength. Thus, for two wavelengths .lambda..sub.1 and .lambda..sub.2, the equivalent wavelength .lambda..sub.eq is equal to: ##EQU1## Ambiguities are thereby reduced by a factor of approximately .lambda..sub.eq /.lambda..sub.1. For very large steps, .lambda..sub.eq must be large and, accordingly, the range is limited by how close together the two wavelengths .lambda..sub.1 and .lambda..sub.2 can be made. This places stringent and, in some cases, effectively unattainable requirements on the necessary accuracy of the two wavelengths used, making this method difficult to implement or impractical for very large steps where typical interferometric precision of several angstroms (.ANG.) is desired.
Coherence scanning methods--as for example disclosed in U.S. Pat. No. 4,340,306 to Balasubramanian, U.S. Pat. No. 4,818,110 to Davidson, and U.S. Pat. No. 5,112,129 to Davidson et al.--involve measuring the coherence envelope or fringe contrast from broadband light in an equal path interferometer while scanning through the equal path condition, or measuring the coherence envelope from narrow band light as is disclosed in copending U.S. patent application Ser. No. 07/893,324. The peak of the coherence envelope, corresponding to the maximum fringe contrast, is then determined as a function of scan or translation stage position. This peak contrast position will reflect changes in the heights of surface features and can thus be used to measure those features. Since the coherence envelope must be inferred from the interference fringes, it is however necessary to densely measure the fringes as a function of scan position. This requirement puts enormous demands on storage requirements for typical data, easily exceeding hundreds of megabytes. Thus, most implementations of coherence scanning perform some type of preprocessing; see, for example, P. J. Caber, "The Use Of Digital Signal Processing Techniques For The Interferometric Profiling Of Rough Surfaces", Masters Thesis in Electrical Engineering, University of Arizona (1991). Unfortunately, in addition to markedly increasing system cost, these procedures substantially reduce the rate at which data is taken, even when using high speed digital signal processors (DSP's) for the preprocessing functions. As a consequence, compromises between speed and data density are required and are typically made in most implementations of coherence scanning, thereby reducing the best available resolutions to the 10-20 angstrom range and scanning speeds to less than 0.5 .mu./sec.
Order counting methods attempt to establish the order of the fringe used in the phase shifting calculation by using contrast information to identify the location of that fringe on the coherence envelope function. Broadband illumination is typically used so that the contrast between adjacent fringes changes sufficiently for a unique determination. The inherent limitation in such procedures is that sources which provide good fringe contrast discrimination have insufficient contrast for use with large steps, and sources which provide enough contrast for large steps lose discrimination for small steps.
Recently-issued U.S. Pat. No. 5,133,601 to Cohen et al. discloses several other related methods and arrangements for the profiling of rough surfaces. Common to each is on-the-fly computation and reconstruction, from the stored intensity data, of the modulation envelope waveform for each pixel so as to determine the derived peak intensity value of the central fringe of the envelope, which peak value is then used to determine the relative phase of the central fringe for each of a multiplicity of imaging pixels for use in calculating a step height. The need to compute the modulation waveform for each imaging pixel places significant computational and data storage requirements on the apparatus and notably slows the rate at which profiling of a surface may proceed and, correspondingly, may limit the attainable precision of the resulting profile measurements.
There is accordingly an unmet need, particularly for use with interferometric coherence scanning microscopes, for an inexpensive, fast and highly accurate method and apparatus for the rapid acquisition of data used in measuring the profiles of surfaces to a precision typically enjoyed by currently known and practiced interferometric methods and apparatus.